The present invention relates to an apparatus for transmitting and receiving ultrasonic signals for an ultrasonic imaging device operating by the pulse echo method.
In the image-giving methods used in medical ultrasonic diagnostics which operate on the pulse echo principle, one evaluates primarily the information of the ultrasonic signals reflected back or scattered back at 180.degree.. Ultrasonic signals which are scattered or reflected at an angle other than 180.degree., however, also contain valuable information about the tissue structure and can contribute to image quality improvement.
From "Ultrasonic Imaging", 3 (1981), pages 62-82, an apparatus for transmitting and receiving ultrasonic signals is known where an ultrasonic transmitter, the trasmitting surface of which forms the curved envelope surface of a truncated cone, annularly surrounds an ultrasonic receiver with circular disk-shaped receiving surface. The piezo-electric element of the ultrasonic transmitter consists of polyvinylidene fluoride, PVDF, as this is suitable, due to its mechanical flexibility, especially for making large-area transducers with a curved surface. By this design of the transmitting surface a focus linearly extended on the axis of the cone is produced. The punctiform tissue structures present on this line focus then produce spherical scattered waves, of which essentially the fraction scattered under the angle of 180.degree.-.theta., where .theta. is the cone aperture angle, gets back into the receiver. By this design and arrangement of the ultrasonic transmitters and receivers a high definition in depth is achieved. By a linear movement of the entire apparatus the line focus sweeps in the body a plane and a two-dimensional sectional image can be produced. With this apparatus, however, the scattering angle under which the ultrasonic signals are received is given by the geometric construction of the ultrasonic transmitter. Discrimination of the ultrasonic image according to different scattering angles is thus not possible during operation.
From "Medical Physics", Vol. 3, No. 5, Sept./Oct. 1976, pages 312-318, in particular pages 317-318, an apparatus for transmitting and receiving ultrasonic signals is known where again the ultrasonic transmitter and the ultrasonic receiver are set up spatially separated from each other. The ultrasonic receiver and ultrasonic transmitter each consist of a plurality of transducer elements. The ultrasonic receiver is a linear array which is surrounded by an annular array serving as ultrasonic transmitter. The individual transducer elements of the annular array are operated with different transmitting power in accordance with their angular position relative to the longitudinal direction of the linear array. The transducer elements, whose connecting line to the center of the annular array lies parallel to the longitudinal direction of the linear array, then emit with maximum intensity of sound. In this manner a sound beam is produced, which is fanned out wide in the plane perpendicular to the transmitting surface and parallel to the longitudinal direction of the linear array, and which is narrow in the direction perpendicular thereto. In addition, the transmitted sound beam as also the focus of the linear array serving as ultrasonic receiver can be pivoted by electronic delay means parallel to the longitudinal direction of the linear array. With this apparatus one obtains good resolution in both lateral directions. With the mentioned apparatus, ultrasonic signals are indeed received from an angle range correlated with the pivoting range of the linear array, but the ultrasonic signal scattered or reflected under different scattering angles are composed to form a joint two-dimensional image.